There are vast deposits of oil shale throughout the world with one of the larger deposits being in the Piceance basin of Colorado, Wyoming and Utah. This oil shale has carbonaceous materials known as kerogen which decomposes on heating to produce shale oil which approximates crude petroleum. The vast oil shale deposits represent a very large source of oil for the world energy economy.
A variety of techniques have been proposed for extracting the shale oil at economical prices. Many of these techniques mine the oil shale by underground or open pit mining and carry it to large retorts where it is heated and the oil extracted. These approaches involve moving massive amounts of material to the retorts and disposing of enormous quantities of spent shale from which the carbonaceous values have been extracted.
Another approach which has significant economic advantages and minimal impact on the environment employs in situ retorting where the shale oil is removed without mining all of the oil shale. Such retorts can be formed, for example, by excavating a portion of rock in a volume that ultimately will become an underground retort. The balance of the rock in the volume to become a retort is then explosively expanded to form a rubble pile of oil shale particles substantially completely filling the retort volume. The original excavated volume is thus distributed through the expanded oil shale particles as the void volume therebetween.
Oil is then extracted from the expanded rubble pile in the underground retort by igniting the top of the rubble pile and passing an oxygen bearing gas, such as air, downwardly through the retort. Once raised to a sufficient temperature the oil shale will support combustion, initially at the top of the retort by burning some of the oil in the shale. Thereafter, as the oil is extracted there is residual carbon left in the shale and, when at a sufficient temperature, this too will react with oxygen to burn and supply heat for retorting. This burning of residual carbon in the shale depletes oxygen from the air being passed down through the retort and the substantially inert gas then carries heat to a retorting zone below the combustion zone for decomposing the kerogen and extracting oil. Gases from the bottom of the retort are collected and often contain sufficient hydrogen, carbon monoxide and/or hydrocarbons to be burnable in heat engines. Oil is also collected at the bottom of the retort and transported for conventional refining.
When the oil shale is expanded in the underground retort, the particles ordinarily fill the entire volume so that there is no significant void space above the rubble pile. Air for combustion can be brought to the rubble pile by means of holes bored through overlying intact rock. Appreciable difficulty may be encountered, however, in igniting the top of the rubble pile to support combustion. Ignition requires a substantial amount of heat delivered over a sufficient time to raise the oil shale above its ignition temperature. Considerable difficulty is encountered in heating a substantial volume of oil shale and assuring that ignition has been obtained.